A dynamic microphone is also called an electrodynamic microphone because voice coils integrally mounted on a diaphragm are disposed in a magnetic gap formed in a magnetic circuit and current is generated on the voice coils by the vibration of the diaphragm as disclosed in, e.g., Patent Document 1 (Japanese Patent Application Publication No. H11-331983). The sensitivity is mostly determined by the magnetic flux density of the magnetic gap, the length of the voice coil, and the velocity of the voice coil.
The length of the voice coil cannot be so large in consideration of an output impedance and a restriction on the volume of the magnetic gap, and thus a design is generally made with 600Ω or lower. Further, the velocity of the voice coil is determined by the design of the acoustic/mechanical vibration system of the microphone unit. Considering an overall directional frequency response, an extremely high velocity is not preferable.
In the dynamic microphone, the magnetic circuit comprises a center pole piece connected to one pole of a permanent magnet and a yoke which is connected to the other pole of the permanent magnet and is arranged like a ring around the center pole piece via a magnetic gap of a predetermined width. The magnetic flux density of the magnetic gap can be increased by reducing the gap width. However, the voice coils are disposed so as to vibrate in the magnetic gap, and thus there is a limit on a reduction in the width of the magnetic gap.
For this reason, a realistic measure to further increase the sensitivity of dynamic microphones has been the use of strong permanent magnets. Thus, neodymium magnets which are compact with a large energy integral are frequently used. Moreover, neodymium magnets contain no expensive metals and thus are readily available at low cost.
However, magnetic circuits have leakage flux to some extent. Particularly in the case of the magnetic circuit used for the dynamic microphone, the magnetic gap for the voice coils is disposed between the center pole piece and the yoke, so that large leakage flux occurs in the magnetic gap.
When the leakage flux of the magnetic circuit is actually calculated with parameters including the outside diameter and thickness of the permanent magnet, the inside diameter of the yoke, and the width, height, and area of the magnetic gap, it is found that leakage flux in the magnetic gap between the center pole piece and the yoke is nearly twice or more than magnetic flux in the other parts of the magnetic circuit.
Therefore, even when a strong permanent magnet such as a neodymium magnet is used for the magnetic circuit of the dynamic microphone, magnetic flux is not effectively used. Hence, there is scope for improvement in the sensitivity of dynamic microphones.